This invention relates to a retardation film, a polarizing plate, and a liquid crystal display device.
A liquid crystal display device uses a retardation film to expand its viewing angle. The retardation film is provided between a polarizer and a liquid crystal cell. Namely, the retardation film is placed between a liquid crystal cell and a polarizing plate obtained by laminating the both surfaces of a polarizer produced by stretching an iodine-doped PVA (polyvinyl alcohol) with protective films made of TAC (triacetyl cellulose). There are various ways of placing the retardation film between a polarizer and a liquid crystal cell according to retardation values and characteristics. For example, the polarizing plate can have only one film on one side or two films on both sides. Further, some films of different kinds in lamination can be placed on one side. In some cases, layers of different optical anisotropic properties can be formed on a single film.
These retardation films respectively have a specific phase difference value (retardation value). The retardation value increases as the film becomes thinner because the film has less gelated portions in film formation. If the retardation value changes, the viewing angle and color of the film vary when viewed particularly obliquely. Various materials are used for the retardation films to enable various film structures. These materials may change reversibly when the environment (particularly humidity environment) changes. When a retardation film has a layer (optical anisotropic layer) which causes another phase difference on the supporting film on which a liquid crystal layer is coated, a TAC film, for example, is used as the support. However, the TAC film itself has a retardation value (particularly along the thickness of the film) and the retardation value changes depending upon moisture absorption and removal. The moisture absorption and removal are affected by the quantity of hydroxy groups in cellulose ester. In other words, the moisture absorption and removal become more vigorous as the total replacement ratio of hydroxy groups goes lower.
Recently, the inventors have produced a film which has both a retardation film function and a function to protect a polarizer by stretching a cellulose derivative film instead of a conventional TAC film to produce a phase difference. (For example, see Japanese Non-Examined Patent Publication 2003-270442.) However, the retardation value of this film changes reversibly when the humidity changes although the degree of the change is lower than that of the conventional TAC film.
Considering the above, it is not avoidable that the total retardation value between the polarizer and the liquid crystal cell changes reversibly when the humidity changes because of the existence of a layer whose retardation value is dependent upon the environment (humidity), including a TAC film which is used as a film to protect the polarizer.
In other words, a problem is that the display quality (viewing angle) of a liquid crystal display device is affected by a retardation change. To solve this problem, the use of a NORBORNENE film was taken into consideration but the NORBORNENE film using water-based adhesives is low in productivity because it is not dried up quickly.
Further, this problem with retardation changes sometimes occurred also when such retardation films and liquid crystal display devices were kept in hot and humid environments during transportation.
Further, since the display quality (viewing angle) of a liquid crystal display device is determined by a retardation value, even if a change of a retardation is small, if the retardation value is not appropriate, there may be a case that a fluctuation of the display quality (viewing angle) of a liquid crystal display device becomes large.